Typically, in wireless communication performed by mobile communication apparatuses, the code division multiple access (CDMA) technology such as the wideband-code division multiple access (W-CDMA) technology is already in widespread use for enabling a plurality of users to perform wireless communication using the same frequency.
An example is explained below with reference to the W-CDMA technology. For each user at the transmitting end, spread spectrum modulation is carried out, in which the target data for transmission is multiplied by an orthogonal code (a spreading code or a channelization code), and then the spread data (channels) is subjected to multiplexing and modulation before being transmitted. In each apparatus at the receiving end, despreading is carried out, in which the received data is multiplied by the orthogonal code used by the corresponding apparatus, so that the data addressed to that apparatus gets demodulated. By performing such operations, the W-CDMA technology enables a plurality of users to perform high-speed communication using the same frequency without having to encounter interference.
In recent years, to enable more number of users to simultaneously perform high-speed communication using the same frequency, wireless communication is implemented in which spreading and despreading is carried out by making use of an orthogonal code as well as a scrambling code (SC). More particularly, at the transmitting end; with respect to each single bit of the target data for transmission, spreading is carried out by multiplication with an orthogonal code assigned to each channel in each cell or each sector. Subsequently, at the transmitting end; with respect to each single bit (chip) of the data that has been spread with the orthogonal code, further spreading is carried out by multiplication with the SC. Then, the spread data is transmitted after being subjected to multiplexing and modulation. In each apparatus at the receiving end, despreading is carried out in which the received data is first multiplied by the scrambling code used by the corresponding apparatus and then multiplied by the orthogonal code used by the corresponding apparatus so that the data addressed to that apparatus is demodulated.
Consider the case of using the scrambling code. For example, in a cellular phone at the receiving end; at the time of initial operations performed when the power is turned ON or at the time of cell searching, an operation is performed for searching the header position of the scrambling code in the data received from, for example, a wireless base station at the transmitting end. In other words, the cellular phone performs an operation for establishing synchronization. More particularly, the wireless base station performs multiplexing of control signals, such as a pilot signal (CPICH: common pilot channel) in which a single frame has a length of 10 msec and includes 15 slots or a synchronization signal (SCH: synchronization channel), and communication data, such as voice data, and then transmits the multiplexed data.
Herein, the pilot signal represents a signal of a certain bit (chip) pattern that is transmitted in a cyclic manner. The synchronization signal represents a signal including a primary-synchronization channel (P-SCH) and a secondary-synchronization channel (S-SCH), which are spreading codes having the same code length of 256 chips in all cells or sectors. The P-SCH is intermittently transmitted for 15 times within a single frame and is used for obtaining the reception timing of the S-SCH. The S-SCH has 16 kinds of codes. One of the S-SCH codes is transmitted at the same timing as that of the P-SCH and is used for obtaining the beginning of the information separated by frames, that is, for obtaining the header position of the SC.
The cellular phone at the receiving end first establishes synchronization by receiving the P-SCH and then obtains the timing of the S-SCH. Subsequently, the cellular phone receives the S-SCH at the timing obtained from the P-SCH and obtains the header position of the scrambling code from the code of the S-SCH as well as identifies the code group of the scrambling code from the patterns of S-SCH codes assigned to the slots in a single frame. Then, the cellular phone receives the CPICH and calculates a correlation between the received CPICH and each scrambling code included in the identified code group of the scrambling code. For example, in the W-CDMA technology, the cellular phone calculates a correlation between the received CPICH and eight types of the scrambling code and specifies the scrambling code having the highest correlation to be the scrambling code of the cell or the sector at which the cellular phone (mobile terminal) is located.
Thus, the mobile terminal makes use of the identified scrambling code for performing despreading with respect to the data received from the wireless base station. Subsequently, using the orthogonal code assigned to each channel in each cell or sector, the mobile terminal performs despreading with respect to the data so that the data addressed to the mobile terminal is demodulated. Meanwhile, in the case of data transmission from mobile terminals to the wireless base station, the data is transmitted using the scrambling code assigned individually to each mobile terminal.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-354255
Nonpatent Document 1: Gregory E. Bottomley, Tony Ottosson, Yi-Pin Eric Wang, “A Generalized RAKE Receiver for Interference Suppression”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 8, AUGUST, 2000
However, in the abovementioned conventional technology; in the case of decoding signals other than a synchronization signal, there are times when interference from the synchronization signal cannot be eliminated. That becomes a particularly significant problem in the case of a fast transmission rate or a high signal-noise (S/N) ratio.
More particularly, in the abovementioned W-CDMA technology, signals other than the synchronization signal are spread using an orthogonal code orthogonal to other codes and a scrambling code thereby eliminating inter-signal interference. On the other hand, the synchronization signal is spread using a code not orthogonal to other codes. Hence, even if the signals other than the synchronization signal are despread using an orthogonal code, the interference from the synchronization signal that has been spread using a code other than orthogonal code cannot be eliminated (canceled). As a result, the interference from the synchronization signal exerts an influence even after performing the despreading. That causes deterioration in the communication quality or in the communication characteristic.
Meanwhile, in the case of a relatively slower transmission rate or a low S/N ratio, the target data for transmission can be transmitted by appending thereto a redundant bit for performing error detection/correction such as forward error correction (FEC). At the receiving end, even in a case when interference from the synchronization signal cannot be eliminated, error detection/correction can be carried out by implementing the FEC so that the interference from the synchronization signal exerts less influence. However, in the case of high-speed transmission using, for example, the high speed downlink packet access (HSDPA) protocol, a redundant bit for error detection/correction can hardly be appended to the target bit for transmission. Consequently, at the receiving end, failure to eliminate the interference from the synchronization signal makes it difficult to carry out accurate error detection/correction. Hence, the interference from the synchronization signal exerts a considerable influence.